This application is based on Patent Application Nos. 2000-261645 filed Aug. 30, 2000 and 2001-215789 filed Jul. 16, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing method and a printing apparatus, and is particularly suited for the application to a printing apparatus by the ink jet method. More particularly, the present invention intends to solve a granular impression of image and a color tone irregularity caused by the ejection order of color inks which are becoming problematic in case where the divided printing, for instance in a bi-directional printing, is performed applying a plurality of times of main scanning according to a pixel arrangement, in a complementary relation for a same image area in a so-called serial type color inkjet printing apparatus. In addition to general printing apparatus, the present invention can also be applied to copying machines, facsimiles with a communication system, word processors with a printer, and industrial printing apparatus combined with a variety of processing devices.
2. Description of the Related Art
An image printing apparatus of so-called serial scan type, which executes the print operation while scanning a print head, or a printing unit, over a print medium, has found a variety of image forming applications. The ink jet printing apparatus in particular has in recent years achieved high resolution and color printing, making a significant image quality improvement, which has resulted in a rapid spread of its use. Such an apparatus employs a so-called multi-nozzle head that has an array of densely arranged nozzles for ejecting ink droplets. Images with still higher resolution has now been made possible by increasing the nozzle density and reducing the amount of ink per dot. Further, to realize an image quality approaching that of silver salt picture, various technologies have been developed, including the use of pale or light color ink with reduced density in addition to four basic color inks (cyan, magenta, yellow and black). A print speed reduction problem, which is feared to arise as the picture quality advances, is dealt with by increasing the number of print elements, improving the drive frequency and employing a bi-directional printing technique, thus realizing a satisfactory throughput.
FIG. 45 schematically shows a general construction of a printer that uses the multi-nozzle for printing. In the figure, reference number 1901 represents head cartridges corresponding to four inks, black (K), cyan (C), magenta (M) and yellow (Y). Each head cartridge 1901 consists of an ink tank 1902T filled with a corresponding color ink and a head unit 1902H having an array of many nozzles for ejecting the ink supplied from the ink tank onto a print medium 1907.
FIG. 46 schematically shows the head unit 1902H in the Z direction for illustrating representing the nozzle array thereof. In this example, ejection openings 2001 are arrayed in one line.
In FIG. 45, designated 1903 is a paper feed roller which, in cooperation with an auxiliary roller 1904, clamps a print medium (print paper) 1907 and rotates in the direction of arrow in the figure to feed the print paper 1907 in the Y direction as required. Denoted 1905 is a pair of paper supply rollers that clamp the print paper 1907 and carries it toward the print position. The paper supply rollers 1905 also keep the print paper 1907 flat and tight between the supply rollers and the feed rollers 1903, 1904.
Designated 1906 is a carriage that supports the four head cartridges 1901 and moves them in a main scan direction during the print operation. When the printing is not performed or during an ink ejection performance recovery operation for the head unit 1902H, the carriage 1906 is set at a home position h indicated by a dotted line.
The carriage 1906, which was set at the home position h before the print operation, starts moving in the X direction upon reception of a print start command and at the same time the head unit 1902H ejects ink from a plurality of nozzles (n nozzles) formed therein according to print data to perform printing over a band of a width corresponding to the length of the nozzle array. When the printing is done up to the X-direction end of the print paper 1907, the carriage 1906 returns to the home position h in the case of one-way printing and resumes printing in the X direction. In the case of bi-directional printing, the carriage 1906 also performs printing while it is moving in a xe2x88x92X direction toward the home position h. In either case, after one print operation (one scan) in one direction has been finished before the next print operation is started, the paper feed roller 1903 is rotated a predetermined amount in the direction of arrow in the figure to feed the print paper 1907 in the Y direction a predetermined distance (corresponding to the length of the nozzle array). By repeating the one-scan print operation and the print paper feeding by a predetermined distance, data for one sheet of paper is printed.
Unlike a monochromatic printing that prints only characters such as letters, numbers and symbols, the color image printing must meet various requirements such as color development, grayscale characteristic and uniformity. As to the uniformity in particular, slight variations among individual nozzles that are produced during the manufacture of a multi-nozzle head formed integrally with many nozzles (in this specification the nozzle generally refers to an ejection opening, a liquid passage communicating with the ejection opening and an element for generating energy used to eject ink) influence the amounts of ink ejected from the individual nozzles and the directions of ink ejection during printing and eventually degrade the image quality in the form of density variations of the printed image.
Detailed examples will be explained by referring to FIGS. 47A-47C, 48A-48C and 49A-49C. In FIG. 47A, designated 3001 is a multi-nozzle head with a construction similar to the one shown in FIG. 46, which is shown to have only eight nozzles 3002 for simplicity. Denoted 3003 are ink droplets ejected from the nozzles 3002. It is ideal that the ink droplets are ejected in equal amounts and in the same direction. If ink ejection is done in this manner, ink dots of equal sizes land on the print medium, as shown in FIG. 47B, resulting in a uniform density distribution with no unevenness in density (FIG. 47C).
In reality, however, individual nozzles have their own variations and if the printing is done in a manner described above, the ink droplets ejected from individual nozzles vary in size and direction as shown in FIG. 48A, forming ink dots on the paper surface as shown in FIG. 48B. From this figure it is seen that a blank part appears cyclically in the head main scan direction, dots overlap excessively in other parts, or a white line occurs at the central part in the figure. The ink dots printed in this way produce a density distribution in the direction of nozzle arrangement or nozzle column as shown in FIG. 48C, which is perceived as unevenness in density by normal human eye.
To deal with the problem of the unevenness in density, the following method has been proposed.
This method will be explained by referring to FIGS. 49A to 49C. Although the head 3001 is scanned three times as shown in FIG. 14A to complete the print in an area similar to that shown in FIGS. 47A-47C and FIGS. 48A-48C, an area of four pixels, one-half the vertically arranged eight pixels, is completed with two scans (passes). In this case, the eight nozzles of the head 3001 is divided into two halves, upper four nozzles and lower four nozzles, and the number of dots formed by one nozzle in one scan is equal to the image data culled to one-half according to a predetermined image data arrangement. During the second scan, dots are embedded at the remaining half of the image data to complete the print in the four-pixel area. This method of printing is called a multi-pass printing method. With this printing method, if a print head similar to the one shown in FIG. 48A is used, the individual nozzle influence on the printed image is halved, so that the printed image will be as shown in FIG. 49B, rendering the white lines or dark lines shown in FIG. 48B less noticeable. Hence, the unevenness in density is significantly improved as shown in FIG. 49C when compared with FIG. 48C.
While the same print area has been described to be completed in two scans, the multi-pass printing improves the image quality as the number of passes increases. This however elongates the print time, which means that there is a trade-off relation between the image quality and the print time.
Under such a situation, there have already been various proposals for the purpose how speedily and beautifully an image can be outputted. Japanese Patent Application Laid-open No. 5-31922 (1993) discloses such contents that an image data arrangement by a tone production method such as a dither method are masked by applying a thinning pattern with dot arrangement asynchronous with the image data arrangement. According to this Application, a data printing ratio is made equal in plural passes as far as possible to obtain a smooth image by using a mask pattern which does not synchronized with a predetermined dither pattern. However, although this method has been able to cope with the predetermined dither pattern for the purpose, it has been difficult to equally cope with all the binarization methods.
Moreover, Japanese Patent Application Laid-open No. 7-52390 (1995) discloses a printing method using a mask pattern provided with randomness. According to this method, the principal object in a divided printing, i.e., improvement in unevenness of an image caused by connecting parts and variation in nozzles is possible to any binarization method.
However, it is difficult to configure a mask applied to a large range such as all over a printing medium without providing a considerable amount of memory; so in practice, it is general to apply a predetermined size of mask to the whole printing medium surface repeatedly. On the other hand, in a printing apparatus, a slight difference is generated for dot formation position between recording scans, as some mechanical errors appear for each recording scan. This slight difference makes the mask pattern applied to respective recording scan visible on the formed image, and sometimes deteriorated the image quality by a reiterative appearance of texture of a predetermined size. Therefore, a configuration for preventing a cyclic repetition of apparent random mask from occurring, by shifting the application start position of random mask having the aforementioned predetermined size for each recording scan, or by randomly changing the shifting amount has been proposed (Japanese Patent Application Laid-open No. 7-125311 (1995)). This makes difficult to recognize visually the deteriorated image quality, because, even if the pattern of the aforementioned predetermined size appears, it is not arranged regularly.
The above-mentioned divided printing has had a problem that time and cost required for printing a sheet of paper increases as the number of division increases, and the throughput of printing decreases. To improve this problem, reduction in printing time can be considered by carrying out printing in the process of reciprocal scanning of a carriage (bi-directional printing). According to this method, since all carriage scanning operations made to return to the home position without printing anything are omitted, a printing time for a sheet of paper can be reduced approximately by half. And, in practice, the bi-directional printing has frequently been adopted as a printing method of a monochrome image.
However, in a color ink-jet printing apparatus, it has been difficult to realize bi-directional printing due to the below described factors.
FIGS. 50A and 50B show a state in which dots of a printing ink widely used at present are landed on a printing medium (paper) P, and here, the figures show the case where ink dots of different colors are absorbed (printed) at almost adjoining positions at a time interval. Here, it should be noted that in the overlapped part of the two dots, the ink dot landed later penetrates and sinks into the paper deeper than the ink dot landed earlier. This fact is for the following reason. Namely, since bonding between the printing medium and a coloring matter is limited at the stage when the coloring matter such as dyestuff in the ejected ink is physically and chemically bonded to the printing medium, and the bonding between the coloring matter in the precedingly ejected ink and the printing medium is prioritized, the ink coloring matter remains on the surface of the printing medium more than the following one, so far as coloring matters do not differ much in the bonding strengths depending on the kinds. Consequently, it is thought that the ink coloring matter subsequently landed is hard to be bonded to the surface of the printing medium, and sinks in the paper in the depth direction to dye and bond it. In this case, even though two kinds of inks are landed at the same position, their colors are prioritized according to the landing order, and result in representing two different colors to visual characteristic of human eyes in case where there are pixels differentiated in the landing order of two kinds of ink even when a mixed color recording is performed with two kinds of ink.
In the construction shown in FIG. 45, the four color heads 1901, which are arranged in order of black (K), cyan (C), magenta (M), and yellow (Y) from the right in this figure, move to the right as shown by the x-coordinate from the printing start position shown in the figure in the forward scanning, and perform printing operation by ejecting each ink in the moving process. Since the order of printing on the paper is in accordance with that of the above-mentioned arrangement in this case, for example, when a signal of green (cyan+yellow) is inputted for a certain area, the inks are absorbed in each pixel in order of cyan and yellow. Therefore, the cyan absorbed precedently is the prioritized color in this scanning, and green dots with a cyan tone are formed. On the other hand, in the return or backward scanning after the paper has been fed in the direction of y-coordinate, the four color heads are positioned at the right side in the figure, and then perform printing operation while moving in the reverse direction of the forward way. Therefore, the landing order is also inverted, and green dots with a yellow tone are formed in this scanning.
If scanning for printing is repeated as the above, a green dot area with a cyan tone and a green dot area with a yellow tone are alternately formed in the sub-scanning direction (y-direction) according to the forward and backward printings with the printing heads. Namely, if print-scanning is carries out without considering the divided printing and the paper is fed by an amount of the y-directional head width between the forward and backward scans, the green area with a cyan tone and the green area with a yellow tone are alternately repeated at each head width in the y direction, and this causes deterioration in quality of the green image which should be even.
However, it is possible to overcome this harmful influence a little by using the divided printing method already described above. Namely, although green dots with a cyan tone are printed in the forward scanning and green dots with a yellow tone are printed in the backward scanning, even if the divided printing is performed, the paper is fed by an amount smaller than the head width between the forward and backward scans, therefore, a color tone in a certain area contains a mixture of both tones of dots, and this relaxes unevenness of color.
These constructions and effects have already been disclosed in U.S. Pat. No. 4,748,453. This invention describes such effects that although a paper feed amount is not restricted, ink is prevented from beading on a medium such as a plastic sheet for an OHP by performing supplementary printing to the pixels alternately positioned in the horizontal and vertical directions in the printing area by printing divided into the first and second (or more) scans, and that when a color image is formed, color-banding (unevenness of color) can be prevented by inverting the landing order of the inks at mixed color pixels in the first and second scans (forward and backward printing). Since it is the principal object of the invention of the above-mentioned Patent to prevent beading between each pixel, it is characterized that each pixel printed in a single scanning is alternate in the horizontal and vertical directions (not adjoining each other).
On the other hand, according to Japanese Patent Application Laid-open No. 58-194541 (1983) applied by the applicant of the present invention, such a printing method is disclosed that when plural arrays of printing elements are arranged in parallel and main scanning of dot matrix printing is carried out by moving them forward and backward in the direction perpendicular to the array of printing elements, the duplicate printing dots by said plural arrays of printing elements have been made to differ in order of the duplicate printing on the forward and backward ways of said main scanning from each other, by intermittently printing the smaller number of dots than the entire dots to be printed at least in either of each row or column of the printing dot matrix in the forward main scanning, and intermittently printing the remaining dots in either of each row or column of the printing dot matrix in the backward main scanning. This Application does not describe either about restriction to reduce a paper feed amount smaller than a normal one as in the divided printing mentioned above, but describes, as an effect, about prevention of image deterioration caused by color tone irregularity (color banding) of a printed image based on the duplicate printing with the color inks. Moreover, since it is the principal object in the invention of the Application to prevent this color tone irregularity, no special restriction is described on dot positions to be printed in each scanning, but horizontal thinning in which dots are alternately printed only in the vertical direction and vertical thinning in which dots are printed alternately only horizontal direction are described, in addition to a checker pattern printing.
Moreover, the Japanese Patent Publication No. 63-38309 (1988) also discloses that, although this Publication is not restricted to a color printer, a construction for performing back-and-forward printing using a diced form (checker pattern) is disclosed. An object of the invention in the Publication is to prevent adjoining dots from being successively printed and to prevent dot-distortion from occurring by avoiding printing an adjacent dot before the printed dot is dried. Therefore, according to this invention, similarly to the above-mentioned U.S. Pat. No. 4,748,453, the thinning-out mask is restricted to a diced form (checker pattern).
However, even though the divided printing is performed according to the checker pattern disclosed in the Publication, the harmful influence of color irregularity is not completely solved yet.
The reason for the above will be explained below by referring to FIGS. 51A to 51C, and 52. Usually, an ink droplet quantity is designed so as to spread larger than an area given to each pixel on the paper. This is for the purpose of completely blinding a white part (ground of a printing medium) of the paper to an area of 100% data printing ratio. Therefore, when a two-divided printing is performed, a pixel itself is printed only 50% by a single scanning, but almost 100% area of the printing medium (printing paper) is covered.
FIGS. 51A and 51B show the cross sections in this case. Here, they show that the first pass (forward scanning) provides a checker pattern printing, and the second pass (backward scanning) provides an inverted checker pattern. FIG. 51A shows the appearances of inks directly after printing in the first pass (forward scanning), and the part fully painted out is printed with cyan ink, and the shaded portion is printed with yellow ink. Since the yellow ink is landed at the same position as the cyan ink at a slight time interval, when they are absorbed in the paper, the cyan ink blurs little and the density stays high, while the yellow ink goes under and around the cyan ink, to blur large, and the density becomes low. Moreover, the absorption of these inks extends to the adjacent pixels in this case, so that the paper surface is almost filled with the inks (FIG. 51A).
In the second pass printing (backward scanning) performed under this condition, the inks are landed on the spot where the adjoining inks are already absorbed. Since the second pass is a backward scanning, the yellow ink is landed before the cyan ink (FIG. 51B). When the inks are absorbed as they are, such an absorption state is brought as both colors does not appear much on the surface in the end as shown in FIG. 51C. And, as a finally completed image, the cyan density of the first printing is emphasized most strongly, and this printing area is provided with a green image with emphasis on cyan. Contrariwise, in the printing area provided with the first pass printing by the backward scanning and adjacent to the above-mentioned printing area, the cyan and the yellow are inverted, so that a green image is obtained with a yellow tone prioritized.
FIG. 52 shows the printed states of the above two printing areas, representing the case in which forward and backward printing was carried out by using a multi-nozzle head with 16 nozzles according to the method described in FIGS. 49A to 49C. From this figure, it can be seen that a precedent half of the head always determines a prioritized color for each area of an eight-dot width, and that the prioritized colors are inverted from each other in forward and backward scanning. Although the description has been made here assuming that a checker patterned mask was applied, a random mask printing method disclosed in the before-mentioned Japanese Patent Application Laid-open No. 7-52390 (1995) has brought a similar result, and since two areas with different prioritized colors existed alternately, color irregularity has still appeared also in divided printing and deteriorated an image, and a bi-directional printing has been made difficult.
As a solution of the harmful effect of the above-described color irregularity, the invention disclosed in Japanese Patent Application Laid-open No. 6-22106 (1994) is mentioned. According to this invention, a group of mxc3x97n pieces of pixels (this group will be referred as a xe2x80x98collective dotxe2x80x99 or a xe2x80x98collective pixelxe2x80x99) is used as a unit for printing, and printing is performed by using an arranging matrix in which the groups are not adjacent to each other. The Application discloses such an effect as an amount swelling out to a blank paper area has been reduced by printing a batch of mxc3x97n pieces of pixels, and a difference between prioritized colors in forward and backward printings has been eliminated to reduce the harmful effect of color irregularity.
As mentioned above, the effect of multi-pass recording has been reflected on the image, by a technique for making the cyclicity of the random mask unobvious while applying the random mask method basically (Japanese Patent Application Laid-open No. 7-125311 (1995)), or a technique for taking a group of mxc3x97n pixels as recording unit, and recording by using an arrangement mask where groups are not adjacent each other.
However, the technique disclosed respectively in the above-described Japanese Patent Application Laid-open No. 7-125311 (1995), Japanese Patent Application Laid-open No. 6-22106 (1994) and Japanese Patent Application Laid-open No. 7-52391 (1995) were not sufficient for recording a high quality photographic tone image which is required in recent years.
Even when the technology disclosed in Japanese Patent Application Laid-open No. 7-125311 (1995) is applied, the presence of texture makes the difference from the silver salt photography evident in a picture composed using a trace of ink so small as several pl (pico-litters) as a unit which is required in recent years. Also, for Japanese Patent Application Laid-open No. 6-22106 (1994) and Japanese Patent Application Laid-open No. 7-52391 (1995), a batch of gathered dots unit for a sufficient effect of controlling color irregularity exceeds a degree of human visual resolution, so that a texture becomes visually conformable. Namely, although the invention has been able to fully cope with a desktop publishing, and a printing for images consist of graphics or text, it could be harmful to a photographic image quality which is in great demand in recent years. Therefore, under the existing circumstances, it is general to cope with the problem by applying a random mask printing method with increasing the number of divided recording, however, when bi-directional printing is carried out to increase the throughput, color irregularity still remains to some extent, the ink landing position deviation between the forward printing scan and the backward printing scan appears as texture, causing the deterioration of image quality.
Moreover, in the bi-directional printing, the deviation between a dot formation position (ink landing position) by the forward scan and a dot formation position by the backward scan is added as a new error factor, and the deterioration of image quality by this also thwarts the photographic image quality. A processing for dot formation position registration (bi-directional registration) of bi-directional scanning in the bi-directional printing can be performed, by applying a technology as described in Japanese Patent Application Laid-open No. 10-329381 (1998) when a user acquired a printing apparatus, or during the use thereafter.
However, even when the bi-directional registration processing is performed a phenomenon in which the print medium surface rises (called cockling, hereinafter) may appear depending on the ink amount afforded in an actual recording operation, and the adjustment appropriate value for the bi-directional registration may become locally different from the other areas. In such a case, the image may impress granular only in the cockled portion, and the granularity becomes all the more confirmable because it is partial. In addition, the state of cockling is also affected by the nature of printing medium, use environment and, further, recording data quantity, and its generation area is uncertain on the printing medium. There, the aforementioned texture appears irregularly in a part of the printing medium surface where there is much undulation, deteriorating the photographic image quality. That is, even the bi-directional registration processing can not cope with the generation of such cockling.
The present invention is made considering the above-mentioned problems, and its purpose is to make it possible to form a high quality image, namely, to make it possible to record images of a photographic quality without granularity nor color irregularity, in case where the divided printing is performed applying a plurality of times of main scanning according to a pixel arrangement in a complementary relation for a same image area.
In a first aspect of the present invention, there is provided a printing method using a print head on which a plurality of ejection openings for ejecting inks are arranged, the method comprising the steps of:
scanning the print head in a main scan direction different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in an sub scan direction perpendicular to the main scan direction by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by a plural times of the main scans in accordance with pixel arrangements in a complementary relation to the same image area; and
applying a mask having data arrangement of less low frequency components and high dispersion to the complementary pixel arrangement, the data arrangement having a visually agreeable arrangement of print permitting pixels and print prohibiting pixels in each of the plural times of main scans.
In a second aspect of the present invention, there is provided a printing apparatus using a print head on which a plurality of ejection openings for ejecting inks are arranged, said apparatus comprising:
means for scanning said print head in a main scan direction different from the arranging direction of said plurality of ejection openings, also relatively transporting said printing medium in an sub scan direction perpendicular to said main scan direction by the quantities less than the arranging width of said plurality of ejection openings, and forming an image on the printing medium by a plural times of said main scans in accordance with pixel arrangements in a complementary relation to the same image area; and
means for applying a mask having data arrangement of less low frequency components and high dispersion to said complementary pixel arrangement, said data arrangement having a visually agreeable arrangement of print permitting pixels and print prohibiting pixels in each of said plural times of main scans.
In a third aspect of the present invention, there is provided a printing system comprising a printing apparatus using a print head on which a plurality of ejection openings for ejecting inks are arranged, and a host apparatus for supplying image data to the printing apparatus, the printing apparatus having:
means for scanning the print head in a main scan direction different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in an sub scan direction perpendicular to the main scan direction by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by a plural times of the main scans in accordance with pixel arrangements in a complementary relation to the same image area; and
means for applying a mask having data arrangement of less low frequency components and high dispersion to the complementary pixel arrangement, the data arrangement having a visually agreeable arrangement of print permitting pixels and print prohibiting pixels in each of the plural times of main scans.
In a fourth aspect of the present invention, there is provided a storage medium storing a control program for making a printing apparatus perform a printing method, the printing apparatus using a print head on which a plurality of ejection openings for ejecting inks are arranged, and the printing method comprising the steps of:
scanning the print head in a main scan direction different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in an sub scan direction perpendicular to the main scan direction by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by a plural times of the main scans in accordance with pixel arrangements in a complementary relation to the same image area; and
applying a mask having data arrangement of less low frequency components and high dispersion to the complementary pixel arrangement, the data arrangement having a visually agreeable arrangement of print permitting pixels and print prohibiting pixels in each of the plural times of main scans.
In a fifth aspect of the present invention, there is provided a control program for making a printing apparatus perform a printing method, the printing apparatus using a print head on which a plurality of ejection openings for ejecting inks are arranged, and the printing method comprising the steps of:
scanning the print head in a main scan direction different from the arranging direction of the plurality of ejection openings, also relatively transporting the printing medium in an sub scan direction perpendicular to the main scan direction by the quantities less than the arranging width of the plurality of ejection openings, and forming an image on the printing medium by a plural times of the main scans in accordance with pixel arrangements in a complementary relation to the same image area; and
applying a mask having data arrangement of less low frequency components and high dispersion to the complementary pixel arrangement, the data arrangement having a visually agreeable arrangement of print permitting pixels and print prohibiting pixels in each of the plural times of main scans.
In any one of the above aspects, the mask may have a pseudo periodical data arrangement.
Here, a step of or means for generating the pseudo periodical data arrangement in a mask area having an extension of the predetermined number of pixels may be comprised, the generating step or means having a first step of or means for setting at random an initial print permitting pixel in the mask area; a second step of or means for affording a predetermined repulsive potential to all of set print permitting pixel positions; a third step of or means for setting the pixel on a position in the mask area where the sum of the potentials becomes minimum as a new printable pixel; and a fourth step of or means for reiterating the processing of the second step or means and the processing of the third or means step until pixels on all positions in the mask area become print permitting pixels.
Alternatively, a step of or means for generating the pseudo periodical data arrangement in a mask area having an extension of the predetermined number of pixels may be comprised, the generating step or means having a first step of or means for dividing the mask area into a plurality of areas in correspondence to the main scan to be performed the plurality of times; a second step of or means for setting at random at least one initial print permitting pixel in at least one the area; a third step of or means for setting pixel of the other area positionally corresponding to the set all print permitting pixels as print prohibiting pixel; a fourth step of or means for affording a predetermined repulsive potential to the positions of all of set print permitting pixels; a fifth step of or means for setting the pixels on a position in the mask area where the sum of the potentials becomes minimum as a new print permitting pixel, except for that set as the print prohibiting pixel; and a sixth step of or means for reiterating the processing of the third step or means to the processing of the fifth step or means until the quotient of the number of pixel in the mask area divided by the number of the areas become print permitting pixels.
The repulsive potential may be expressed by a function attenuating according to a distance from the print permitting pixel.
In any one of the first to fifth aspects, the data arrangement may have blue noise characteristics.
Further, in any one of the first to fifth aspects, the data arrangement applied at least to a first main scan among the plurality of times of main scans may be specified in a collective pixel having a size of m pixel(s) in the main scan direction and n pixel(s) in the sub scan direction (m and n are integrals. Provided that at least one of them is equal or superior to 2) as unit.
Here, the data arrangement may be a pseudo periodical data arrangement of the collective pixels.
The m may be an integer equal or superior to 2, and adjacent the collective pixels are arranged each other, so as to overlap in the sub scan direction in a range equal or superior to 1 pixel and less than m pixels.
Alternatively, the n may be an integer equal or superior to 2, and adjacent the collective pixels are arranged each other, so as to overlap in the main scan direction in a range equal or superior to 1 pixel and less than n pixels.
In any one of the first to fifth aspects, a plurality of the ejection openings may be arranged for a plurality of inks different in color tone, and the data arrangement applied to one time of the main scan may be differentiated according to the color tone.
In any one of the first to fifth aspects, at least two times of the main scan may be performed respectively in the forward and backward directions for the same image area, in the processing by the image formation step means, the sum of the data arrangements applied to the at least two times of main scan in the forward direction, and/or the sum of the mask arrangements applied to the at least two times of main scan in the backward direction may be also arrangements presenting little low frequency components and a high dispersion, and having a visually agreeable arrangement of print permitting pixels and print prohibiting pixels.
Alternatively, in any one of the first to fifth aspects, at least two times of the main scan may be performed respectively in the forward and backward directions for the same image area, in the processing by the image formation step means, the sum of the mask arrangements applied to the at least two times of main scan in the forward direction, and/or the sum of the mask arrangements applied to the at least two times of main scan in the backward direction may be pixel arrangements specified by a collective pixel having a size of m pixel(s) in the main scan direction and n pixel(s) in the sub scan direction (m and n are integrals. Provided that at least one of them is equal or superior to 2) as unit.
The sum of the data arrangements applied to the at least two times of main scan in the forward direction, and/or the sum of the data arrangements applied to the at least two times of main scan in the backward direction may be also arrangements presenting little low frequency components and a high dispersion, and having a visually agreeable arrangement of print permitting pixels and print prohibiting pixels.
In any one of the first to fifth aspects, data arrangement does not synchronize with a mask arrangement of the time when an image input as printing object may be binarized.
Further, in any one of the first to fifth aspects, the plurality of ejection openings may be arranged on the print head in the sub scan direction with an interval of the transport pitch multiplied by an integer, the image formation step or means may perform the image formation by performing the plurality of times of main scan and the transport, and the application step or means may apply a visually agreeable data arrangement of little low frequency components and high dispersion, to the pixel arrangement to be printed by one time of the main scan.
The invention according to any one of the above aspects may further comprises a step of or means for controlling, in the image formation, the sum of ratios of data print quantity in the main scan of odd-number of order among the plurality of main scans to the total data print quantity for the same image area be smaller than the sum of ratios of data print quantity in the main scan of even number of order among the plurality of main scans.
Here, the ratio of data print quantity in the first main scan among the plurality of main scans may be made smaller than the ratio of data print quantity in the second main scan.
The main scan may be performed three times or more for the same image area, the ratio of data print quantity in the third main scan and thereafter may be made larger than the ratio of data print quantity in the first main scan, and smaller than the ratio of data print quantity in the second main scan.
The sum of covering ratio of the print medium by dots formed in the first main scan and the second scan may be made larger than 50%.
The data arrangement applied at least to the first main scan among the plurality of times of main scan may be specified taking a collective pixel having a size of m pixel(s) in the main scan direction and n pixel(s) in the sub scan direction (m and n are integrals. Provided that at least one of them is equal or superior to 2) as unit.
In any one of the first to fifth aspects, further comprising the step of controlling, in the image formation, so that the sum of ratios of data print quantity in the first main scan among the plurality of main scans be smaller than the ratio of data print quantity in the second main scan, and the sum of covering ratio of the print medium by dots formed in the first main scan and the second scan be larger than 50%.
Here, the covering ratios of the print medium by dots formed in the first main scan and the second scan may be made approximately equal.
The invention according to any one of the above aspects may further comprises:
a first control step of or mode for controlling, in the image formation, so that the sum of ratios of data print quantity in the main scan of odd number of order among the plurality of main scans to the total data print quantity for the same image area be smaller than the sum of ratios of data print quantity in the main scan of even number of order among the plurality of main scans; and
a second control step of or mode for performing a control similar to the first control step or mode for a plurality of times of main scans less that the first control step, and specifying the arrangement pixel in at least the first main scan among the plurality of times of main scan taking a pixel group of m pixel(s) in the main scan direction and n pixel(s) in the sub scan direction (m and n are integrals. Provided that at least one of them is equal or superior to 2) as unit.
Here, the changeover selection of the first control step or mode and the second control step or mode may be made possible.
In any one of the above aspects, the plurality of times of main scan for forming an image on the print medium includes the main scan in the forward direction and the main scan in the backward direction.
Further, in any one of the above aspects, the print head may have a plurality of columns of the plurality of ejection openings, in correspondence to inks different in color tone, juxtaposed in the direction of the main scan.
Moreover, in any one of the above aspects, the print head may have heating elements to generate thermal energy for causing film boiling in ink as an energy for ejecting ink from the ejection openings.
According to the present invention, in a serial type ink jet printing apparatus using a print head where a plurality of ink ejection openings are arranged and completing an image on a printing medium by performing a bi-directional print scanning in a direction perpendicular to the arrangement direction of the plurality of ink ejection openings, while feeding paper successively in a direction parallel to said plurality of ink ejection openings, even when a longitudinal or transversal deviation of ink landing position occurs, a stable image without granular impression can be obtain, by applying a divided printing method for completing an image on a printing medium by a plurality of printing scans using a plurality of pixel arrangements where pixel arrangements made of print permitting pixels and print prohibiting pixels are in a mutual complementary relation, wherein the pixels arrangement printed on the print medium by a single print scan is made to become visually agreeable arrangements (for example, pseudo periodical arrangement).
Also, according to the present invention, it becomes possible to record rapidly an image with photographic quality exempt from irregularity of color by the bi-directional printing, by making the sum of proportion of data print amount in the scanning of odd number of order among said a plurality of forward and backward scans for the data print amount of the whole smaller than the sum of proportion of data print amount in the scanning of even number of order among said a plurality of forward and backward scans, or, by making the proportion of the data print amount in the first scan among said a plurality of forward and backward scans smaller than the proportion of the data print amount in the second scan. Further, a smoother print with less image perturbation becomes feasible, by introducing a pseudo periodical arrangement mask making the arrangement of print permitting pixels and print prohibiting pixels visually more preferable.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.